A group III-V compound semiconductor material containing nitrogen (hereinafter referred to as “nitride semiconductor material”) has a band gap equivalent to the energy of light having a wavelength from the infrared region to the ultraviolet region. Therefore, the nitride semiconductor material is useful as a material for a light-emitting element that emits light having a wavelength from the infrared region to the ultraviolet region or as a material for a light-receiving element that receives light having a wavelength in the same region, for example.
Further, the nitride semiconductor material possesses a strong interatomic bonding force, a high dielectric breakdown voltage and a large saturated electron velocity. Therefore, the nitride semiconductor material is also useful as a material for an electronic device such as a high-frequency transistor which is resistant to high temperatures and has a high output power. Furthermore, since the nitride semiconductor material almost does no harm to the environment, and thereby it is gaining attention as an easy-to-handle material.
In an nitride semiconductor light-emitting element made of the nitride semiconductor material having the above characteristics, it is common to employ a quantum well structure in a light-emitting layer. When a voltage is applied to the nitride semiconductor light-emitting element employing the quantum well structure in the light-emitting layer, electrons and holes in the quantum well structure of the light-emitting layer are recombined to generate light. As the light-emitting layer having a quantum well structure, it is acceptable to adopt a single quantum well (SQW) structure, but in most cases, a multiple quantum well (MQW) structure in which quantum well layers and barrier layers are stacked alternately is used instead.
Generally, a quantum well layer of a light-emitting layer is made of InGaN, and a barrier layer thereof is made of GaN. Thus, it is possible to fabricate a blue LED (Light-Emitting Device) having an emission peak wavelength of about 450 nm, and it is also possible to fabricate a white LED in combination with a yellow phosphor. In the case where the barrier layer is made of AlGaN, it is believed that the luminous efficiency may be improved due to the increase on the difference of band gap energy between the quantum well layer and the barrier layer, but it is accompanied by such a problem that it is harder to obtain crystals of good quality from AlGaN than GaN.
Generally, an n-type nitride semiconductor layer is made of GaN or InGaN.
For example, Japanese Patent Laying-Open No. 2004-343147 (PTD 1) discloses an LED element having a structure where an n-side multi-layered film layer which includes a nitride semiconductor layer containing In is disposed below an active layer. According to the LED element described in PTD 1, it is inferred that the n-side multi-layered film layer disposed below the active layer functions to improve the output of the light-emitting element by improving the crystallinity of the active layer.
Japanese Patent Laying-Open No. 2002-299685 (PTD 2) discloses a group III nitride LED in which a spacer layer and an active region are deposited in order on a smoothing layer doped with Si in the range of 2×1017 cm−3 to 2×1019 cm3, and the smoothing layer is doped much heavily than the spacer layer. In the group III nitride LED of PTD 2, the smoothing layer has a function of restoring a flat two-dimensional growth of the group III nitride semiconductor layer at a low temperature, and thereby, both the efficiency and the reliability of the group III nitride LED can be improved.
Japanese Patent Laying-Open No. 2005-203520 (PTD 3) discloses a light-emitting diode which is provided with a buffer layer made of Si-doped GaN semiconductor, a third AlGaN semiconductor layer 9 made of Si-doped Al0.18Ga0.82N semiconductor and a light-emitting region composed of well layers 35a to 35c and barrier layers 37a to 37d both made of InAlGaN semiconductors on a supporting substrate composed of a GaN substrate, and emits light having a peak wavelength of 359 nm.
Japanese Patent Laying-Open No. 9-153645 JP (PTD 4) discloses a light-emitting diode which includes by stacking a high carrier concentration n+ layer having a film thickness of about 2.0 μm and made of silicon-doped GaN having an electron concentration of 2×1018/cm3, an n layer having a film thickness of about 1.0 μm thickness and made of silicon-doped Al0.3Ga0.7N having an electron concentration of 2×1018/cm3, a light-emitting layer having a total film thickness of about 0.11 μm, a p layer having a film thickness of about 1.0 μm and made of Al0.3Ga0.7N doped with magnesium having a hole concentration of 5×1017/cm3 and a magnesium concentration of 1×1020/cm3, and a contact layer having a film thickness of about 0.2 μm and made of GaN doped with magnesium having a hole concentration of 7×1017/cm3 and a magnesium concentration of 2×1020/cm3 on an AlN buffer layer, and has a light-emitting peak wavelength of 380 nm.
Japanese Patent Laying-Open No. 10-173231 (PTD 5) discloses a light-emitting element structured to have a Si-doped layer n+GaN with a carrier concentration of 1×1018/cm3 and an n-type In0.15Ga0.85N layer grown in order on a Si-doped n+GaN layer with a carrier concentration of 1×1019/cm3. In the light-emitting element described in PTD 5, a current flows uniformly in the entire active layer, and thereby, uniform light emission can be obtained.    PTD 1: Japanese Patent Laying-Open No. 2004-343147    PTD 2: Japanese Patent Laying-Open No. 2002-299685    PTD 3: Japanese Patent Laying-Open No. 2005-203520    PTD 4: Japanese Patent Laying-Open No. 9-153645    PTD 5: Japanese Patent Laying-Open No. 10-173231